A liquid-metal fast-breeder reactor (LMFBR) uses a large vessel for holding a core within which fission of nuclear materials takes place. A primary coolant, typically molten sodium, is circulated through the core to cool it. An intermediate coolant (generally sodium also) cools the primary coolant by means of a sodium-sodium intermediate heat exchanger (located within the reactor vessel for a pool type system or located outside of the reactor vessel but close by in the same building in a loop type system). A thermodynamic working coolant (water/steam) cools the intermediate coolant by means of a sodium-water heat exchanger (steam generator) located in another building. The steam formed within the steam generator is conveyed via piping to a conventional steam turbine for powering an electric generator.
Thus, the conventional sodium cooled fast reactor power plant requires costly equipment interposed between the reactor core (where nuclear fission takes place), and the steam turbine (which utilizes the thermodynamic heat generated by the nuclear fission). The coolants are maintained separated from one another, other than for thermal heat transfer contacts across the respective heat exchangers, and are circulated by pump means in closed loop coolant systems. The closed loop for the primary coolant of sodium includes the reactor core and the intermediate heat exchanger, with piping and sodium pumps constituting major components. The closed loop for the intermediate coolant of sodium includes the intermediate heat exchanger, the steam generator, piping, pumps, expansion and drain tanks, and purification equipment, and most of these components are located outside of the reactor vessel. The closed loop for the thermodynamic working coolant, or steam system, includes the steam generator, steam turbine, piping, pump, condensors, water purification equipment, and feedwater heaters, and all of these components are located outside of the confinement of the reactor vessel.
The sole purpose of the intermediate coolant loop is to assure that the sodium in the steam generator-steam turbine loop will not be radioactive. For safety reasons, it is considered necessary to separate this water/steam coolant from the radioactive sodium coolant by two barriers, to minimize the possibility of a radioactive fire that could occur otherwise in the event a leak allowed these coolants to contact one another. Conventionally, these two barriers consist of the tube walls in the intermediate heat exchanger and in the steam generator.
One form of heat exchanger is known as a "heat pipe". In the heat pipe, a sealed structure holds a working medium that transfers the heat from a vaporizing section (in heat transfer relationship with a heat source) to a condensing section (in heat transfer relationship with a heat sink). A conventional heat pipe is elongated and the heat source and heat sink are located at opposite ends of the pipe, whereupon the working medium within the pipe evaporates at one end, travels axially along the pipe as a vapor, condenses at the other end, and returns as a liquid on the inner walls of the pipe by gravity or capillary action to the vaporizing end. The heat transfer capacity of such an arrangement is related to the cross sectional area of the heat pipe.
Various proposals have been made to utilize heat pipes as a means for withdrawing heat from the fission reaction and transferring the same to the water/steam coolant system. Heat pipe cooling systems for fission reactors have typically proposed using a separate vessel, apart from the reactor vessel, for holding the heat pipes; but this involves duplicated containment means and appreciably adds to the overall cost. The reason for the need for using a separate containment vessel is in part due to the limited heat transfer capacity per heat pipe, being related to the cross sectional area as noted, where inadequate cooling capacity was provided where the heat pipe cooling systems fitted into the reactor vessel, necessitating therefore a similarly unattractive alternate solution involving increasing the physical size of the reactor vessel to provide the needed space within the reactor for the heat pipes. Also, it is not advisable, for safety reasons, to penetrate the sides of the reactor vessel below the sodium level, which limits alternate design variations of heat pipe cooling systems.